An understanding of neural function, both normal and abnormal, requires an understanding of mechanisms of synaptic interaction. Particularly needed is information about mechanisms of transmitter release and regulation of release,--information that has been difficult to obtain because of the paucity of preparations in which presynaptic events can be monitored and manipulated directly. This proposal describes a series of experiments taking advantage of a preparation in which this can be done- - the synapses formed by motoneuron varicosities on muscle cells in embryonic Xenopus nerve-muscle cultures. In this preparation, patch electrodes can be used to record and stimulate both pre- and postsynaptically, correlating presynaptic currents with transmitter release. Major emphasis will be on (a) characterizing the types of Ca++ channels present in presynaptic varicosities and the degree of coupling of each to transmitter release; (b) exploring the mechanisms whereby Ca++-activated K+ channels (K-Ca) enhance release; (c) using the K-Ca channels as monitors of the changes in Ca++ concentration near release sites as a function of time after Ca++ channel opening; (d) determining the degree of cooperativity of Ca++ ions in release, and the characteristics of Ca++ buffering/extrusion in developing terminals; (e) exploring the development of evoked quantal release mechanisms and the basis for evoked release of only a subset of quantal sizes; (f) testing hypotheses to explain the mechanism of action of alpha-latrotoxin, the Black Widow spider toxin that evokes massive quantal release in a way that appears to bypass the normal release process; and (g) studying mechanisms of modulation of release both by endogenous substances such as adenosine, norepinephrine and metenkephalin, and the enhancement and competitive suppression of release mediated by feedback from target cells during development and maturation. The Xenopus culture preparation is ideally suited for exploring mechanisms of control of transmitter release from nerve terminals; and the proposed experiments should provide a much more detailed understanding of these mechanisms than currently exists.